Fuel cells directly transform chemical energy to electrical energy by reacting electrochemically gas or liquids in the presence of an electrolyte, electrodes and a catalyst. Our previous U.S. Pat. No. 4,673,624 xe2x80x9cFuel Cellxe2x80x9d, U.S. Pat. No. 5,631,099 xe2x80x9cSurface Replica Fuel Cell, and U.S. Pat. No. 5,759,712 xe2x80x9cSurface Replica Fuel Cell for Micro Fuel Cell Electrical Power Packxe2x80x9d described a method of forming a fuel cell that efficiently utilizes expensive catalysts, is easily mass produced, and can be packaged for portable electronics.
A variety of methods for production and/or delivery of hydrogen gas into fuel cells are known. Some of them include pressurized hydrogen storage in cylinders and storage into metal hydride alloys, such as those used in Nixe2x80x94MH rechargeable batteries.
Needs exist to provide safe and convenient sources of hydrogen fuel for fuel cells at a low cost, especially in portable power applications.
A different way of creating and delivering this hydrogen is to use chemical hydride compounds that absorb water or other liquids or gases which react with the chemical hydride to form hydrogen. The hydrogen then diffuses out and is delivered to the fuel cell or hydrogen consuming device.
There are a variety of chemical hydrides which release hydrogen when combined with water. Their reaction with water can be described by the following general equation:
xe2x80x83MHx+xH2Oxe2x86x92M(OH)x+xH2
where M is a metal of valence x. Examples of these chemical hydrides include LiH, LiAlH4, LiBH4, NaH, NaAlH4, NaBH4, MgH2, Mg(BH4)2, KH, KBH4, CaH2 and Ca(BH4)2 (Kong et. al). Kong et al. showed that LiH, LiAlH4, LiBH4, NaH, NaBH4 and CaH2 all deliver a large fraction of their hydrogen capacity upon reaction with water vapor. Conversely, LiBH4 and NaBH4 were observed not to react with water vapor and there was no reaction with water until the powders were effectively dissolved. The U.S. Army Mobility Equipment Research and Development Command developed a system in which liquid water flows from a reservoir into a chamber where it contacts a porous hydrophobic membrane. In this system, water vapor diffuses through the membrane and spontaneously reacts with the hydride to release hydrogen, which then flows out of the reaction chamber to the anode of the fuel cell. Hydrogen production is controlled by virtue of water being forced back into the water reservoir during periods of no load, when the hydrogen is not being consumed.
As another example, Millennium Cell has developed a chemical hydrogen generator based on basic solutions of sodium borohydride (Amendola et al.). The generation of hydrogen is based on the reaction:
NaBH4+2H2Oxe2x86x92NaBO2+4H2xe2x80x83xe2x80x83(1)
Basic solutions of sodium borohydride were shown to be stable. A catalyst (Ru) releases hydrogen when in contact with the solution and therefore requires mechanical means to bring the solution into contact with the catalyst. This adds complexity to the system.
This last case is a particular example of the more general case where NaBH4 and water are chosen from a larger class of compounds A and B, respectively.
This new invention addresses the pre-existing problems.
A subject of this invention is to advance fueling systems such as described in our U.S. Pat. No. 5,759,712 that has the fuel ampoules sealed in gas tight packages. An advance is to arrange a system of two ampoules or components (A and B) such that when placed together produce hydrogen and when separated do not. Diffusion regulation mechanisms are used to regulate the production rates, as well as the choice of both A and B components. An objective is to make hydrogen fueling system safer for hydrogen consuming systems such as fuel cells. In products, an A ampoule and a B ampoule are placed in a cavity and sealed together. Reactants diffuse through the walls of one of the ampoules into the other ampoule through the walls of the second ampoule. The reaction then produces hydrogen gas which diffuses out of the second ampoule.
Obvious applications of a small fuel cell are in those that are currently powered by batteries, especially rechargeable batteries. By safely encapsulating intrinsically energetic fuels with an interactive hydrogen release reaction, the fuel cells can have higher energy per unit mass, higher energy per unit volume, and be more convenient for the energy user.
Our invention provides a safe, convenient, inexpensive and portable hydrogen generator which can be used to fuel a PEM fuel cell. Component A is chosen from chemical hydrides such as LiH, NaH, NaBH4 CaH2 and LiAlH4, among others. Component B may include, but is not limited to, substances such as water, alcohols, organic and inorganic acids (e.g. acetic acid, sulfuric acid), aldehydes, ketones, esters, nitrites and superacids (e.g. polyoxotungstates), and combinations thereof. Depending on the choice of component B, an appropriate selectively permeable membrane should be selected (e.g. silicone rubber for methanol).
Our recently issued patent U.S. Pat. No. 6,194,095 describes how the non-bipolar fuel cells can be packaged to form larger power supplies. Our pending patent no. U.S. Ser. No. 09/821,053 describes an ampoule of fuel that can be delivered at a controlled and constant rate by using the selective permeability of the fuel tank. In some fuel cells a controlled release of hydrogen or other gas is also needed.
This patent describes a recipe where the choice of A and B influences the rate of hydrogen gas generation. More importantly, this patent discloses a method for combining both chemicals without the aid of any mechanical means, thus resulting in a chemical hydrogen generator which is safe, portable and inexpensive.
In our patent application No. U.S. Ser. No. 09/821,053, a liquid hydride solution is immobilized and its contact is controlled with capillary wicking material. It does not have the feature of two components in diffusion contact, instead describing physical contact of a single fuel with a catalyst. Capillary wicking can be used to immobilize any liquid reactants in a two component diffusion delivery system.
In our patent U.S. Pat. No. 5,759,712, a vapor phase transport to a hydrophilic outer surface of a gas manifold is described. Selectively permeable membranes in proximity to the fuel cell are described for delivering reactants and products. Fueling is done by breaching a fuel tank and wicking fuel, which is then transported to the fuel cells in the vapor phase. Breaching this fuel tank can lead to spilling of fuel while liquid contact needs to be maintained with the fuel in the fuel tank. Thus, as the fuel tank runs low on fuel some of the fuel may not be in liquid contact and will be unused. To achieve wicking fuel delivery, the fuel needs to be fluid and mobile thus increasing the possibilities of leakage from the fuel ampoule. Gravity can affect the delivery of a liquid fuel. Achieving a good liquid seal on methanol fuel can lead to complex and costly sealing mechanisms for the fueling system and the fuel cell system. Small leaks of liquid fuel compared to vapor loss through the same hole can have a far greater detrimental effect on the air electrode and total fuel loss.
In our U.S. Pat. No. 6,326,097 the fuel cell and fueling ampoules are shown being placed in proximity to each other with a diffusion mat. The fuel tanks are described as a liquid wick or fluid motion fueling. Fuel diffusion from the fuel tanks is not described. Plastic blister packaging of the fuel tanks does not indicate the sealing properties of the package, nor individual sealing. Porous fillers are described as being in the fuel tanks, but not as diffusion delivery means.
Hydrogen Gas Generation
Chemical hydrides are known to react with water and give off hydrogen gas as a product. Reaction (1) shown above serves as an example. In general, hydrogen generation occurs when the hydride ion reacts with a proton from another source. Water is the most common source of protons used, and hence its reactions with chemical hydrides have been extensively studied and are well documented. Additives which reduce the pH of the aqueous solution result in a higher rate of hydrogen generation. Conversely, raising the pH to an appropriate level can stabilize sodium borohydride solutions, effectively lowering the reaction rate to the point where almost no hydrogen evolves.
Other reactants may be used instead of water. For example:
NaBH4+4CH3OHxe2x86x92Na[B(OCH3)4]+4H2xe2x80x83xe2x80x83(2)
We have carried out this reaction in our laboratories and found that the rate of hydrogen generation in the methanolysis reaction is considerably higher than in the hydrolysis reaction. As an example, the powder form of NaBH4 can be mixed as a slurry in a silicone rubber mix to be encapsulated. Water or methanol diffuse through the silicone rubber to the NaBH4 and the generated hydrogen can diffuse out. An encapsulated cylinder with water or methanol and a separate encapsulated NaBH4 cylinder could be placed together in the fuel manifold of a fuel cell to generate hydrogen. We have found this vapor delivery system at room temperature and conditions to be very slow and unsteady with water as the reactant. However with methanol vapor it is immediate and steady. We have tested the produced gas in a residual gas analyzer and confirmed that the product gas is hydrogen, as expected from reaction (2).
This reaction system allows us to package the reactants as an AB system of two ampoules which separately do not make hydrogen at a significant rate. We have also found that ethanol and isopropanol do not significantly react with NaBH4.
Our pending patent application U.S. Ser. No. 09/821,053, the disclosure of which is incorporated herein by reference, discloses a method of using a selectively permeable membrane to diffuse selected chemical species (e.g. methanol through silicone rubber) to provide fuel for a fuel cell. That invention is herein expanded by including a separate component, namely a chemical hydride contained in an ampoule, which reacts which said diffused species producing hydrogen gas which can then be used to fuel a PEM fuel cell.
Other reactions may be used to generate hydrogen. Alkaline, alkaline earth metals and metals from Groups IIIA and IVA may react with water, alcohol or dilute acids to liberate hydrogen. The metals may be encapsulated in a matrix and the reactant (water, methanol, dilute acid) allowed to diffuse in through a selectively permeable membrane.
The reaction of the hydrides with water produces basic solutions and compounds. When the product is a strong base this can be neutralized with carbon dioxide to form a carbonate and water. Other alternatives are the reactions with acids. This makes available more water for the reaction with the hydride and neutralizes the caustic base. The carbon dioxide could come from the direct methanol or hydrocarbon utilizing fuel cell. In some cases by making water available from the reaction this would be a net gain in the energy per unit mass for the system. An ideal combination might be a hydrocarbon which results in hydrogen gas and a carbonate product when combined with the chemical hydride. A fueling system is made up of two diffusion ampoules which slowly diffuse into the other to produce hydrogen gas and solid encapsulated carbonate products when placed within an enclosure. As an example, consider the reaction of water with lithium hydride:
LiH+H2xe2x86x92LiOH+H2xe2x80x83xe2x80x83(3)
A subsequent reaction takes place between LIOH and CO2:
LiOH+CO2xe2x86x92Li2CO3+H2Oxe2x80x83xe2x80x83(4)
The water resulting from (4) can react with unreacted LiH as shown in reaction (3) to further produce hydrogen gas. At the same time, a more benign end product (lithium carbonate) is produced.
In our previous patents U.S. Pat. No. 5,631,099 and U.S. Pat. No. 5,759,712, the carbon dioxide exhaust can diffuse out through a selectively permeable membrane or the fuel tank itself. This feature can control reaction (4). We have found that a simple gaseous diffusion route such as a capillary tube can effectively exhaust product gases and maintain gas pressure equilibrium across the fuel cells. The capillary tube can also function as a controlled leak for the beneficial in-leakage of oxygen to the fuel electrode.
The production of fuel such as hydrogen from a chemical hydride can be regulated by delivery of moisture or acid. A valve or pump can used to regulate moisture from a tank containing the moisture to the second tank containing the hydride. This is an AB system where the two tanks work together. The moisture can be also very useful to maintain the humidity for the fuel cells. Thus all the separate parts can form an interacting system. The moisture valve can be a membrane with small controllable apertures, or a gap between the tanks that is increased to decrease the delivery of reactants and is decreased to increase the delivery of reactants.
Hydrogen ion-drag pumping through a membrane can be used to drag water and solvents from one ampoule to the other. This can act as a solid state pumping system to move reactants. The amount of reactant moved across the membrane is proportional to the electrical current going through the membrane. Thus, the production of hydrogen can be controlled through the electrical current through the separating membrane. Examples of a suitable membrane are Nafion with platinum-catalyzed electrode on either side.
Another feature is to add a separate catalyst mixed with the hydride to increase the hydrogen production rate. Noble metal catalysts (e.g. Pt, Ru) and transition metals in general are particularly suitable to perform this function. Other suitable catalysts are substances such as Co2B, CoCl2, CuCl2, NiCl2, Fe2 when mixed with NaBH4 powder.
Selectively Permeable Membranes
By having a selectively permeable fuel tank wall, the fuel delivery can have the advantageous effect of delivering fuel at a constant rate throughout its life cycle. Component B may be made up of two or more chemicals, one or more of which may react with the chemical hydride. If the membrane had similar permeability to the main chemical in component B (i.e. the fuel) compared to a minority chemical in component B, the latter would diffuse in while the former diffused out. The presence of this minor chemical would drop the fuel vapor pressure and reduce the rate at which fuel can diffuse out. Thus, the rate of fuel delivery would gradually drop. As an example, our measurements on silicone rubber membranes show a molecular diffusion rate difference for methanol over water of 20 to 36 times. In performance tests with a small ampoule containing 95% methanol and 5% water with a silicone rubber membrane, the fuel delivery system is effective in delivering fuel with only a small fraction of the original fuel volume left as water in the fuel container.
Mixed fuels in component B made up of chemicals such as methanol, formaldehyde, formic acid and water could be used in the fuel ampoule. If these additives are permeable through the fuel ampoule they will also be delivered according to their respective rates and concentration gradients. The fuel ampoule material could also be chosen or designed by a mixture of materials to have a permeability that allows the fuel to be delivered at the rate ratio matching that of the fuel. An example is an ampoule wall material that has a 1:1 diffusion rate for methanol over water. Thus, if fueled by a 1:1 fuel mixture and assuming a low exterior concentration of both, the diffusion delivery system would deliver fuel at a 1:1 concentration.
The process of enhancing the selective vaporization of fuel from a membrane is called per-evaporation. It essentially increases the evaporation of that fuel. The ampoule membrane may use this effect when the fuel concentration is low. It can keep the fuel concentration higher at the fuel cell than it would be without the fuel ampoule selectively permeable membrane.
The tank walls can be made of composite materials. Examples are fiberglass cloth and silicone rubber, where the fiberglass cloth gives mechanical strength and the silicone rubber has high diffusion rate properties. The mechanical and diffusion properties of the fuel tanks can be adjusted to reflect the blend of materials and components making up the fuel system. The tanks may also be made in layers. One option is to make the outer layer have the highest diffusion resistance and have a single fuel such as methanol, with the interior having rapid diffusion. This would give the fuel delivery a flat output with time, matching the vapor pressure of the fuel liquid, and then a steep decline as the remaining vapor diffuses out of the interior materials and voids.
Incorporating electrical and mechanical diffusion control into the fuel ampoule or between the fuel ampoule and the fuel cell allows the membrane diffusion to have a feedback mechanism to adjust to consumption demands by the fuel cell, or to different environmental conditions around the system. Possible mechanisms are drawing fuel using ionic drag through a membrane, piezoelectric operating of micro apertures in the membrane, or impermeable membranes that act as apertures which can be adjusted to a specific opening path between the fuel ampoule and the fuel cell, or alternatively a fan.
The permeability of the fuel ampoule can vary with temperature. This property can be used to match the fuel cell consumption rate as the temperature increases. The permeability rate can also be chosen to not rise as much as the fuel cell consumption rate to keep the fuel cell at higher temperatures using more fuel than necessary. This could be the case in power applications where the power delivery is constant regardless of the temperature environment.
Molecular filtration can be used to keep impurities that can be dissolved in the fuel or come with the fuel to be left in the fuel ampoule. This feature can be used to allow a fuel of low purity. The fuel cell may also be protected by using the same principle. The hydrogen gas generated inside the ampoule with the chemical hydride may be filtered using a selectively permeable membrane made of palladium or an alloy thereof before the hydrogen fed to the PEM fuel cell. This keeps impurities from affecting fuel cell performance.
The vapor fuel delivery and selective permeability of the ampoule also have the effect of filtering the fuel. Additives such as dyes, flame colorizers and bitterants can be added to the fuel to make the fuel safer and possibly aesthetically pleasing to consumers. Adding water absorbing chemicals can be added to the fuel to maintain the vapor pressure of the fuel. The interior of the tank could have a filler, such as cellulose sponge, that has a higher diffusion rate to fuel than the walls but would keep liquid fuel from being accessible even if the fuel tank is ruptured or crushed.
By simply being able to remove the fuel tank from a sealed container and sliding it into a chamber without alignment necessities, a system with large dimensional tolerances where the user can close the cover is very simple and makes it convenient and low error prone.
The fuel tank as it uses fuel, if it has selective fuel delivery, will mechanically collapse. This fuel tank collapse can be used to form a mechanical fuel status indicator. A color stripe could be used that moves by a viewing window the fuel ampoule could be used. The fuel tank itself can be tinted to give a visual indication of fuel level. The fuel can have colored dyes so that as the fuel is used it will give a color change indication of fuel status since the remaining fuel will be darker. The fuel ampoule can also have materials, such as salts, that produce an opaque interior or color change in the fuel ampoule as the fuel is used.
Safety
An important feature of our invention is that the chemical hydrides, traditionally thought of as dangerous can be safely immobilized in a number of ways.
The first is to contain the reactants inside a porous bag or material. The powdered hydrides are contained within a hydrophobic porous plastic such as microporous polypropylene. In the event that the bag is dropped in water, only vapor contact with the hydride is made. The liquid fuel can be held in a container that has porous walls that will gradually wick the fuel to the surface of the container.
The second is to employ a container which can be a wicking sponge material such that the chemical hydride or fuel are distributed through it.
Another way is to employ a container which can have a pore free material that surrounds the liquid or solid fuel. Delivery of reactants would be by diffusion through this material and may be selective, e.g. such as the preferential permeability of silicone rubber to methanol over water.
Yet another method employs an ion-exchange membrane. The fuels could be reacted with an ion exchange material making them chemically attached to a surface or polymer. Both A and B fuels could be held by the ion exchange materials.
Packaging
The next challenge is to package the diffusion system to work with fuel cells or other devices and maintain the desired flow rate. A potential problem is that the delivery rate of component B to the ampoule which contains component A will be uneven depending on where the individual particles of the latter are within their ampoule. Molecules of component B will react first with those particles in the adjacent ampoule closest to the selective membrane. This may have the effect that the rate of hydrogen production decrease with time. To compensate for this, the diffusion wall encapsulated materials or homogenous material composites can be perforated with small channels. The diffusion walls can also have their highest resistance concentrated at the surface. We have found in experiments that the encapsulation can gradually break apart as the reaction proceeds opening up further in diffusion routes. A fan or pump system that forces moist gas or fluid through the material can be used to increase the interaction of the water and the hydrides. With a feedback loop to this fan the output of the generator can adjust the production rate to match the consumption rate. The small perforation hydrophobic pores in silicone rubber keep liquid water from being able to penetrate and increase the production rate. The encapsulation can also be designed to have an outer skin that is the predominant rate limiting diffusion barrier and where the interior of the encapsulation has a relatively high diffusion rate.
An unusual design of a reactor that mixes various features of the above description and our previous patent No. U.S. Ser. No. 09/821,053 is to encapsulate the dry reactant in the form of a long capillary tube or tube bundle and a liquid filled capillary tube or tube bundle. Each of these capillary tubes can form a separate ampoule. When the two ampoules are pressed together contact is made with a liquid filled capillary tube and the dry reactant ampoule. Diffusion of liquid and direct liquid contact is forced into the dry hydrophobic reactant tubes by static pressure on the back of the liquid reactant tubes. The liquid reactant will diffuse into the walls of the tube and produce hydrogen which can diffuse back out through the walls of the tube. Bubbles will form and grow in the liquid and reactants and drive a water droplet through the long capillary tube until the droplet is broken. The flow of produced hydrogen will continue to push forward the liquid reactant vapor. A small pump or static pressure against the gas pressure in the back of the liquid capillary tube can control the reactant delivery. When the evolved gas pressure is high, it will force the liquid back out of the dry reactant capillary tubes to the source tube. This design does not have the capability to fully shut off the reaction due to the diffusion though the capillary tubes, but if either reactor has a long diffusion length of non reactant between each other this can be effective in reducing the reaction to a low rate.
The storage container of the permeable fuel container needs to be impermeable to the fuel. This container could be a disposable bag with metal coatings or coatings such as Aclar(copyright) (Honeywell Specialty Films, PO Box 1039, 101 Columbia Road, Morristown, N.J. 07962) PVDF polyvinylidene fluoride plastic. This tank could also be made of composite materials, such as PET plastic polyethylene terephthalate with an Aclar coating. The storage container could be a heat sealed bag with a tear point to allow the consumer to easily open. The essential parallel is that of packages for foods and ink jet cartridges.
These and further and other objects and features of the invention are apparent in the disclosure, which includes the above and ongoing written specification, with the claims and the drawings.